1. Field of the Invention
The present invention relates to a toner and a cleaning method using the same which are available for hard copy apparatus such as a copying machine and a printer.
2. Description of the Prior Art
At present, in an electrophotographic apparatus widely used in the copying machine and laser printer, fields it is well known to form a sheet of toner image on a plain paper by a method in which, after the formation of an electrostatic latent image on an electrostatic latent image carrier body such as an electrophotographic photoconductor, for example, the image is developed by a charged toner electrostatically and is then transferred onto plain paper by means of a corona transfer device which is applied with a voltage of a polarity opposite to that of the toner.
Examples of such apparatus include an apparatus which has been proposed by the present inventors in Japanese Patent Unexamined Publication No. 2-46474. The outline of this apparatus will be described by referring to FIG. 2.
Developing units 1, 2, 3 are non-magnetic one-component developing units of non-contact type in each of which a toner is made to jump in a DC electric field. Conductive fur brushes 4, 5, 6 which are in contact with developing rollers serve to charge frictionally the toners, and blades 10, 11, 12 serve to form thin layers of toners on the aluminum developing rollers 7, 8, 9, respectively. The developing units 1, 2 and 3 hold insulating toners of yellow (Y), magenta (M) and cyan (C), respectively. A black developing unit 13 is a contact type developing unit which is widely used in the electrophotographic apparatus and holds a two-component developer composed of an insulating toner and a magnetic carrier. The developing units are arranged around a photosensitive medium 15 keeping a space between the developing rollers 7, 8, 9, 14 and the photosensitive medium 15. Each developing unit is equipped with a mechanism by means of which it is moved close to and away from the photosensitive medium 15 at the time of developing and non-developing, respectively.
An amorphous Se-Te photosensitive drum 15 serving as the photosensitive medium is charged to an electric potential of +900 V by a charger 16. Subsequently, a semiconductor laser 17 emits light to expose negative black signals onto the photosensitive medium 15 to form an electrostatic latent image. The latent image thus formed is reversed-developed by the black developing unit 13 in the developing condition that the developing roller 14 is applied with a voltage of +600 V, thus forming a black toner image. Then, the charge of the photosensitive medium 15 is once discharged by an AC corona charger 18. Subsequently, the photosensitive medium 15 is charged again to +600 V by the corona charger 16. Thereafter, the semiconductor laser 17 emits light to expose signals corresponding to yellow onto the photosensitive medium 15 to form an electrostatic latent image of yellow. Then, the photosensitive medium is made to pass through the yellow developing unit 1 in the developing condition that the developing roller 7 is applied with +600 V, as well as the magenta developing unit 2, the cyan developing unit 3 and the black developing unit 13 which are in the non-developing condition, thus forming a yellow toner image. Next, the charge of the photosensitive medium 15 is discharged by the AC corona charger 18 and, thereafter, the photosensitive medium 15 is charged again to +810 V by the corona charger 16. Then, the semiconductor laser 17 emits light to expose signals corresponding to magenta to form an electrostatic latent image of magenta. Subsequently, the photosensitive medium 15 is made to pass through the yellow developing unit 1 in the non-developing condition and the magenta developing unit 2 in the developing condition that the developing roller 8 is applied with +800 V, thus forming a magenta toner image. Thereafter, the photosensitive medium 15 is made to pass through the cyan developing unit 3 and the black developing unit 13 which are in the non-developing condition. Then, after the charge of the photosensitive medium 15 is discharged by the AC corona charger 18, the photosensitive medium 15 is charged again to +850 V by the corona charger 16. Thereafter, the semiconductor laser 17 emits light to expose signals corresponding to cyan to form an electrostatic latent image of cyan. Subsequently, the photosensitive medium 15 is made to pass through the yellow developing device 1 and the magenta developing unit 2 which are in the non-developing condition, and through the cyan developing unit 3 in the developing condition that the developing roller 9 is applied with +830 V, thus forming a cyan toner image. In this way, a color image is built up on the photosensitive medium 15.
A sheet of plain paper 19 is conveyed on a transfer belt 21 while being in contact with a fur brush 20 of stainless steel which is applied with a voltage of +1 kV, and made to pass through between the brush and a paper attraction charger 22 to be brought into close contact with the transfer belt 21. After the color toner image formed on the photosensitive medium 15 is transferred onto this paper 19 by means of a transfer charger 23, the paper is charged by a paper detach charger 24 and then separated from the transfer belt 21. The paper thus separated is made to pass through between a pair of chargers including a positive charger 25 and a negative charger 26 so as to be charged and, in addition, made to pass through a fixing unit 27 for ensuring the thermal fixing.
The electrophotographic process in which a plurality of color toner images superimposed on the photosensitive medium are transferred to the paper simultaneously suffers from a problem in that a large quantity of toner remains on the photosensitive medium after the transfer operation. The quantity of the remaining toner in this process is greater than that in the conventional monochrome type. To cope with this, in this apparatus, a conductive fur brush which is applied with a DC voltage is used to attract and remove the remaining toner by a strong electrostatic force. This method, however, suffers from a problem in that not only is a larger and more complex driving device required for rotating the fur brush but also the toners are scattered as the fur brush rotates.
For this reason, there has been made an attempt to adopt a magnetic brush cleaning method which has conventionally been used in part in the monochrome copying machine, making use of the feature of this color electrophotographic apparatus that the color toners are developed in a non-contact manner but the black toner is developed in a contact manner. This magnetic brush cleaning method is one wherein a bias voltage having a polarity opposite to that for development is applied to a two-component magnetic brush developer used for the development so as to attract and remove electrostatically the toner remaining on the photosensitive medium. As a result, not only is the apparatus simplified and reduced in size, but also very satisfactory cleaning results were obtained so far as the apparatus was used under ordinary conditions.
However, there arose another different problem under high humidity conditions. Namely, the color toners of yellow, magenta and cyan are one-component toners which are charged frictionally with the fur brushes, while a black toner is the two-component toner which is charged frictionally with the carrier, and therefore, mixing of the one-component color toners into the two-component black developer caused the color toners to be charged frictionally again with the carrier to be made gradually to have a polarity opposite to their inherent charged polarity under high humidity conditions, resulting at last in a poor cleaning performance.